The basic functions of radar are inherent in the word, which stands for radio detection and ranging. Measurement of target angles is also included as a basic function of most radars and doppler velocity is often measured directly as a fourth basic quantity. Resolution of the desired target from background noise and clutter is a prerequisite to detecting and measurement, and resolution of surface features is essential to mapping or imaging radar.
The radar resolution cell is a four-dimensional volume bounded by antenna beam widths, width of the processed pulse and band width of the receiving filter. Within each such resolution cell, a decision may be made as to presence or absence of a target and, if the target is present, the position may be interpolated to some fraction of the cell dimensions.
A typical radar system includes a transmitter subsystem, an antenna subsystem and a receiver and signal processor subsystem. The radar system also includes a synchronizer which controls the time sequence of transmissions, receiver gates and gain settings, signal processing and display. When called for by the synchronizer, a modulator of the transmitter applies a pulse of high voltage to an RF amplifier, simultaneously with an RF drive signal from an exciter of the radar system. The resulting high power RF pulse is passed through a transmission line or a wave guide to a duplexer which connects it to the antenna subsystem for radiation into space.
Many radar antennas are of the reflector type and are steered mechanically by a servo-driven pedestal. Alternatively, a stationary array may be used with electrical steering of the radiated beam.
After reflection from a target or object, the echo signal reenters the antenna which is connected to a preamplifier or mixer of the receiver subsystem by the duplexer. A local oscillator signal furnished by the exciter of the system translates the echo frequency to an intermediate frequency (I.F.) which can be amplified and filtered in the receiver prior to more refined signal processing. The processed I.F. signal is passed through an envelope detector and displayed with or without video processing.
Any device that detects and locates objects or targets by radiating electromagnetic energy and uses the echo scattered from the target can be classified as a radar no matter what its frequency. Radars have been operated at wave lengths of 100 meters or longer to wave lengths of 10.sup.-7 meters or shorter. The basic principles are the same with any frequency, but the implementation is widely different. In practice, most radars operate within the microwave frequency range, but there are many notable exceptions.
A set of letter designations exist for the frequency bands commonly used for radar. Original code letters (P, L, S, X, and K) were introduced during World War II. After the need for secrecy no longer existed, these designations remained. Others were added later (C, K.sub.u and K.sub.a) as new bands were opened and some were seldom used (P and K). The K-band has a frequency range of 18 to 26.5 GHz.
Radar targets or objects consisting of multiple scattering elements whose phase relationships cause fluctuations in signal amplitude are subject to errors in radar position measurements. The apparent source of the composite echo signal wanders back and forth across the target, and at times the signal appears to originate from points well beyond the physical spread of the target itself. In principle, the variance in position measurement is infinite for a measuring system with unlimited dynamic range and bandwidth. However, for practical systems this target "glint" and "scintillation" error is closely approximated by an gaussian distribution.
One way of camouflaging objects or targets from radar is to use various combinations of optical and radar camouflage. Radar camouflage typically is made from material having multiple layers of lossy dielectric and supporting fabric. Optical camouflage typically is made from material having no special electrical properties but is colored to blend with surrounding scenery. In both cases, the material is cut to produce a random pattern of openings or apertures.